Optical storage systems record digital data onto the surface of a storage medium, which is typically in the form of a rotating magnetic or optical disc, by altering a surface characteristic of the disc. The digital data serves to modulate the operation of a write transducer (write head), which records binary sequences onto the disc in radially concentric or spiral tracks. When reading this recorded data, a read transducer (read head), positioned in close proximity to the rotating disc, detects the alterations on the medium and generates a sequence of corresponding pulses in an analog read signal. These pulses are then detected and decoded by read channel circuitry in order to reproduce the digital sequence. When the pickup (read/write) head of the computer storage system operates, a light beam emitted by a light source such as a laser diode is focused by an object lens on the rotating disc so as to realize the information carried thereby.
Referring to FIG. 1, a conventional tracking control device 10 for controlling tracking operation of an optical pickup head (or, an actuator) 11 comprises a pre-amplifier 12, a compensator 13 and a power amplifier 14. An optical disc 1 is often rendered eccentric in the manufacturing process. In addition, when the optical disc is loaded into an optical disk drive and then clamped by a spindle motor, the center optical disc might be eccentric from the center of the optical disk drive, resulting in a certain degree of runout R while rotating. During operation, an error signal e between the position P of the optical pickup head 11 relative to the disc 1 and the runout R is processed by the pre-amplifier 12 to generate a tracking error TE. If the tracking error TE is substantially zero, it means the optical pickup head 11 has precisely locked the target track, and will acquire correct data. In order to converge the tracking error TE to zero, the generated tracking error TE is transmitted to the compensator 13, e.g. a digital signal processor (DSP), to be processed. The compensator 13 operates on the tracking error TE to generate a tracking output signal TRO for the shift control of the optical pickup head 11. The power amplifier 14 then magnifies the generated tracking output signal TRO for actuating the optical pickup head 11 to move along the current tracking direction. The position information of the optical pickup head 11 relative to the disc 1 is then detected and transmitted to the pre-amplifier 12 again, and the above procedures are repetitively executed until the tracking error TE is lowered to an acceptable level (substantially zero).
As is understood by those skilled in the art, the performance of an optical disk drive, including quality and speed, depends largely on the tracking operation of the optical pickup head 11. For example, the pickup rate of the optical disk drive will be adversely affected if the above-mentioned tracking operation has to repeat a number of times to lock the correct track.
In addition to the eccentricities, some parameters such as the gain variations of different power amplifiers and the moving sensitivity variations of different optical pickup heads (both of the variations may depend on manufacturing processes or element degenerations) might also cause the increase of tracking operation time because they are not taken into account in advance in the conventional tracking control method.